1. Field of the Invention
The present invention relates to a method utilized in a wireless communication and communication device thereof, and more particularly, to a method of managing an offset operation of discontinuous reception in a wireless communication system and communication device thereof.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
A radio interface protocol of the LTE system includes three layers: the Physical Layer (L1), the Data Link Layer (L2), and the Network Layer (L3), wherein a control plane of L3 is a Radio Resource Control (RRC) layer, and L2 is further divided into a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer and a Medium Access Control (MAC) layer.
The main services and functions of the MAC layer include, discontinuous reception (DRX), mapping between logical channels and transport channels; multiplexing/demultiplexing of RLC PDUs belonging to one or different radio bearers into/from transport blocks (TB) delivered to/from the physical layer on transport channels; buffer status reporting; power headroom reporting; error correction through HARQ; priority handling between logical channels of one UE; priority handling between UEs by means of dynamic scheduling; and padding.
The UE may be configured by the RRC layer with the DRX functionality that allows the UE to discontinuously monitor a PDCCH (Physical Downlink Control Channel) carrying information about resource allocation of a DL-SCH (Downlink Shared Channel) and uplink (UL) scheduling grants. The DRX functionality is operated based on the following parameters that can be configured by a DRX-configuration information element (IE) of the RRC layer. The related DRX parameters includes:
A DRX Cycle providing a periodic repetition of a “On Duration” followed by a possible inactivity period, where the DRX Cycle is a short or a long DRX cycle;
A DRX Inactivity Timer providing the number of consecutive PDCCH-subframe(s) after successfully decoding a PDCCH indicating an initial UL/DL user data transmission;
A DRX Retransmission Timer providing the maximum number of consecutive PDCCH-subframe(s) as soon as a DL retransmission is expected by the UE;
A DRX Short Cycle Timer providing the number of consecutive subframe(s) during which the UE shall follow the short DRX cycle after the DRX Inactivity Timer has expired;
A HARQ RTT Timer providing the minimum amount of subframe(s) before a DL HARQ retransmission is expected by the UE; and
An On Duration Timer providing the number of consecutive PDCCH-subframe(s) at the beginning of a DRX Cycle. The UE monitors the PDCCH only during the On Duration.
When the DRX operation is configured, the UE starts the On Duration Timer if the short DRX cycle is used and [(SFN*10)+subframe number] modulo (the short DRX cycle)=a DRX Start Offset, or if the long DRX cycle is used and [(SFN*10)+subframe number] modulo (long DRX cycle)=the DRX Start Offset. The SFN is a system subframe number of a cell used by the UE.
As can be seen from the above, the same “DRX Start Offset” parameter is applied for managing starting of the short and long DRX cycles. Besides, the DRX-configuration IE includes a “longDRX-CycleStartOffset” IE that contains configuration values of the long DRX cycle and the DRX Start Offset
Normally the long DRX cycle is configured to be longer than the short DRX cycle. Therefore, the long DRX cycle Start Offset is possibly incompatible with the short DRX cycle.
For example, the long DRX cycle is 2560 subframes, the short DRX cycle is 640 subframes, and the DRX cycle Start Offset is 1000. Using the DRX cycle Start Offset to configure the DRX Start Offset for the short DRX cycle is unreasonable because no subframe can meet the following equation:[(SFN*10)+subframe number] modulo 640=1000
In this situation, the On Duration Timer of the short DRX cycle is impossible to be started.